Various mechanisms have been developed for use in meters to translate flow of a fluid, such as water or oil, to a measurable quantity. One such mechanism is an ultrasonic flow meter.
The typical prior art ultrasonic flow meter positions pairs of ultrasonic transducers on a segment of pipe or conduit, with one transducer located upstream and the other downstream with respect to the direction of fluid flow in the pipe. Each pair of ultrasonic transducers typically sends and receives an ultrasonic pulse, or series of ultrasonic pulses, back and forth. That is, the first transducer in the pair generates a pulse, or series of pulses, which is received by the other transducer. The time of flight of each pulse, or the average time of flight of the pulses in the series of pulses, is measured. The second ultrasonic transducer then sends a pulse, or series of pulses, to the first transducer. Again, the time of flight is measured. The fluid flow causes the pulses traveling downstream (i.e., with the fluid flow) to move faster, and those traveling upstream (i.e., against the fluid flow), to move slower, than the speed of sound in the static fluid. Thus, as is known in the art, the rate of flow can be determined based upon the difference in flight time between the pulses moving downstream and those moving upstream. Because the speed of sound in a fluid is dependent on the temperature of the fluid, accuracy of the meter can vary with temperature if the meter is not calibrated to temperature.
Prior art meters suffer from a number of disadvantages. The arrival time of each pulse must be detected with high accuracy, if the meter is to be accurate. This requires timing precision much smaller than the period of the ultrasonic signal if acceptable resolution is to be achieved, which causes the meters to be expensive. Also, it can be difficult to determine exactly what is the beginning of the pulse, as received by the receiving transducer. This requires high bandwidth as well, making the system susceptible to noise. Calibration to temperature may require static flow conditions to determine the speed of sound at a given temperature, and also may be based upon factory settings and conditions that are difficult to reproduce in the field.
Thus, there exists a need for a meter in which can automatically calibrate in response to temperature changes and operates in a relatively narrow band, allowing simplified electronics and increased resistance to noise. The present invention satisfies these needs by providing a method and apparatus including an ultrasonic meter with two transceivers that transmit and receive a continuous tone in a narrow band simultaneously, with continuous feedback allowing for automatic adjustment of the transmitted signal to accommodate temperature variations.